This invention relates to a process for the preparation of alkyl-ureas by the direct reaction of alkyl-amines with urea.
N-alkyl- and N,N-dialkyl-ureas, particularly N,N-dimethyl- and N,N-diethyl-urea are known. These compounds provide particularly interesting precursors and intermediate products for the production of isocyanates, active substances, polyurethanes and rocket propellants (as N,N-dimethylhydrazine, J. Russ. Phys. Chem Soc. 37 1).
Various preparations, such as a reaction of an appropriate carbamic acid chloride with ammonia, the reaction of a salt of an appropriate amine with an alkali metal cyanate or the reaction of an appropriate amine with a nitro-urea, have been suggested (Houben-Weyl, Georg Thieme Verlag, Stuttgart 1952, Volume VIII, 153). The starting materials used are relatively expensive, physiologically hazardous and difficult to obtain. Preparation by the reaction of an alkyl- or diaklyl-amine with urea has also been attempted. In U.S. Pat. No. 2,253,528, the process is either carried out in a closed reaction vessel under excess pressure or, if the boiling point of the amine is the same as or higher than the reaction temperature, it is carried out in an open system. It has also been suggested that the reaction be carried out in a complex countercurrent apparatus under pressure as far as possible (German Auslegeschrift No. 1,111,613). Alternatively, it has been proposed to introduce a gaseous amine into the molten urea (German Pat. No. 896,640). Since it is known that urea, when molten, is prone to secondary reactions (e.g. biuret formation), the last mentioned prior art process suffers from the disadvantage that only impure products are obtained and in a poor yield.
According to German Pat. No. 855,551 or U.S. Pat. No. 3,937,727, the reaction between urea and methyl- or dimethyl-amine takes place under pressure in an aqueous or anhydrous medium. These known processes, which take place under external pressure, suffer from the disadvantage that autoclaves or complicated apparatus have to be used.
An object of the present invention was to provide a process which avoids the above-mentioned disadvantages of the known processes. That is, in particular, a process which, regardless of the boiling point of the amine used, allows the production, under atmospheric pressure, of unsymmetrical alkyl-ureas of high purity and in a high yield so that the industrial production of these compounds may be greatly simplified.
Surprisingly, this problem may be solved by the concomitant use of certain diluents in the preparation of unsymmetrical alkyl-ureas by the reaction of urea with aliphatic mono- or diamines. The solution according to the present invention of the prevailing problem may be said to be surprising as it was not to be expected that the concomitant use of diluents of this type would reduce the tendency of urea toward the formation of undesired secondary products.